Fuel cycle technology has recently been adapted to aid in solving nuclear waste disposal problems. For example, weapons grade plutonium blended with uranium is converted into mixed oxide fuel to be burned in commercial light water reactors. Weapons grade plutonium, however, which is comprised of a plutonium-gallium (Pu--Ga) alloy, has presented significant problems when utilized directly as a reactor fuel, because the gallium, typically about 1 to 3 atom % of the Pu--Ga alloy, deleteriously effects fuel element-cladding chemistry. Therefore, development efforts have focused on processing Pu--Ga alloys to separate the gallium from the plutonium, prior to using the weapons grade plutonium in the fuel cycle.
Electrorefining processes are used to recover high purity metals from metal alloys containing impurities, such as plutonium from spent nuclear fuel. Electrorefining is performed in an electrochemical cell in which the fuel elements or fuel alloys form the anode, and a molten eutectic salt electrolyte is used to transfer select ions formed at the anode and discharged as purified metal to the cathode. The fuel elements may be dissolved in a liquid metal anode pool that is immiscible with the electrolyte, or disposed in a moveable basket for immersion in the electrolyte. FIG. 1 shows a prior art electrochemical cell (LANL design) having a non-conducting ceramic electrolyte container 16 containing a molten electrolyte salt 18 (preferably a chloride salt), a solid cathode cylindrical shell or ring 20 positioned within the electrolyte 18, and an anode made of impure molten plutonium 12 (e.g., Pu--Ga alloy) and contained in an inner ceramic crucible 14. The liquid anode is stirred by a ceramic agitator 24. During operation, the plutonium metal from the liquid anode pool is oxidized to form a salt (e.g., PuCl.sub.3) at the anode-electrolyte interface, and the salt is transported within the electrolyte and reduced to plutonium metal at the cathode. The electrorefining process is operated above the melting point of plutonium, such that the purified liquid plutonium drips off the cathode ring and collects at the bottom 22 of the electrorefining cell in the annulus region.
By using the electrochemical cell shown in FIG. 1, approximately 90% of the plutonium metal is removed from the anode and recovered as purified cathode metal before the anode solidifies, preventing stirring of the anode and thus terminating the operation of the electrochemical cell (i.e., for a Pu--Ga alloy initially comprised of 1 wt % gallium, the anode solidifies after about 85% to 90% of the plutonium has been removed from the anode). The remaining 10% of the plutonium is part of the solidified ring of anode. The solidified anode, also referred to as the heel, is a process waste made of impure plutonium (about 25 atom % Ga and about 75 atom % Pu) and must be subjected to further processing. Additional problems include migration of impurities toward the salt-anode interface where the plutonium oxidation is occurring, and limits on the solubility of the impurity within the liquid plutonium. For example, in the case of a gallium impurity, the Pu--Ga intermetallic compound blocks or clogs the liquid salt-liquid metal interface and prevents further electrolysis. Although stirring the anode with a ceramic stirrer 24 increases the amount of liquid plutonium at the salt-anode interface, breakage of the stirrer 24 occurs at a high frequency, after the Pu--Ga anode begins to solidify.
Another electrorefining process for separating plutonium from gallium involves dissolution of the Pu--Ga alloy in a liquid cadmium anode. The Pu--Ga alloy is dissolved in the liquid cadmium anode of the electrochemical cell, and plutonium ions are transported by an electrolyte to a solid cathode where solid plutonium metal dendrites form, or, alternatively, to a liquid cadmium cathode. This cadmium diluted anode electrorefining process operates at a temperature of about 500.degree. C., well below the melting point of plutonium, and is a single phase liquid system for all compositions. Where a liquid cadmium cathode is used, pure plutonium is recoverable by distillation. This approach requires little stirring, since the salt-metal pool anode interface does not become blocked with solids; after repeated cycles, however, gallium accumulates in the gallium-cadmium (Ga--Cd) anode pool, and the gallium to cadmium ratio in the anode is great enough to inhibit dissolution of Pu--Ga alloys. The Ga--Cd liquid anode is then retorted to separate the volatile cadmium from the non-volatile gallium. The cadmium condensate is recycled, and the gallium is recovered and may be used for producing Pu--Ga alloys.
Electrorefining spent fuel containing gallium has significant corrosion and process limitations. For example, ferrous metal is vigorously attacked by intergranular diffusion and compound formation caused by the gallium in temperatures between about 400.degree. C. and 500.degree. C. Gallium also forms very strong compounds with uranium, plutonium, and alkali, alkaline earth, rare earth, and some noble metals. Gallium effects distribution coefficients and separation factors, and, although gallium does not oxidize into salt, its alloying properties also effect electrorefining separation processes used to recycle spent fuel. Therefore, it is desirable to separate the gallium from the plutonium prior to using Pu--Ga alloys to produce fuel.
The invented method for separating gallium from Pu--Ga alloys disclosed herein uniquely employs a solid anode for significantly improving the overall rate of electrotransfer by four to ten times the state of the art. The method involves moving a solid Pu--Ga alloy anode in liquid electrolyte at a temperature sufficient to cause a liquid layer to form on the surface of the solid Pu--Ga alloy (e.g., 500.degree. C.). The surface plutonium is oxidized and effectively stripped from the surface layer until only gallium essentially remains. The plutonium is electrotransported to a cathode and recovered in a purified form. The liquid gallium drips from the surface and is collected at the bottom of the electrorefining cell.
Therefore, in view of the above, a basic object of the present invention is to provide an efficient method for separating impurities from plutonium.
Another object of this invention is to provide an electrorefining method for separating plutonium and gallium from a Pu--Ga alloy, wherein essentially pure plutonium is recovered.
Another object of this invention is to provide a one step electrorefining method for separating plutonium and gallium from a Pu--Ga alloy that does not produce a waste product containing plutonium, requiring further processing.
Yet another object of this invention is to provide a method for processing a Pu--Ga alloy that overcomes anode solidification problems experienced in the prior art.